Anodizing of aluminum for architectural purposes is a high-energy-consuming process. A typical architectural anodizing machine requires tens of kilo-amperes. Ordinarily, an anodizing run in a water solution of sulfuric acid at room temperature lasts 25 min to form a 10-micron-coating on aluminum, or 50 min for a 20-micron-coating. Following Faraday's Law, the current density in these conditions does not exceed 1-1.5 A/dm.sup.2 for different aluminum alloy compositions.
Said range of current densities can be achieved using a DC power supply with the voltage control from 0 to 17-25 VDC. Experience shows that within this voltage range the heat dissipation in the oxide film is still below the level causing catastrophic dissolution of the oxide film and of the aluminum it covers. This catastrophic dissolution is often called "burning". Without the danger of burning, the air agitation of tank electrolyte may be moderate. Besides, no reduction of electrolyte temperature below the room temperature level is needed.
An oxide film formed in these conditions is rather soft--it can easily be scratched. The film is porous, and therefore it can be used as a base to hold coloring agents. A coloring agent can be either an organic dye introduced into the coating by an additional process, or an inorganic substance introduced into the coating at the second step of the architectural anodizing process known as a "two-step" process.
On a much smaller scale, when current consumption is below the 10 kA level, a process analogous to the architectural anodizing described above is called "conventional" anodizing process.
Another kind of coating, called "hard coating" is formed in a water solution of sulfuric acid at higher current densities: over 2 A/dm.sup.2 versus 1-1.5 A/dm.sup.2 in conventional or architectural anodizing. Hard coating has a sapphire, or close to it, hardness that distinguishes this coating from a much softer "conventional" coating. Typically, the hard-coating process forms thicknesses in excess of 50 microns which are often used to change dimensions of aluminum articles. A hard-coating run may last 40 minutes and more, depending on required thickness. A hard-coating process is conducted at much higher voltages than in conventional anodizing. The voltage can reach 70 VDC and more. Multiplying this voltage by the current density of 2 A/dm.sup.2 and more, we arrive at power levels that dramatically exceed the power spent in conventional anodizing.
The electric power sent to a hard-coating tank dissipates mostly in the oxide film formed on aluminum articles. To prevent "burning", the air agitation of the electrolyte must be vigorous and the electrolyte temperature must be dropped to 0.degree. C. and below. A typical hard-coating machine requires hundreds amperes to several kilo-amperes depending on productivity of the machine. A 5,000 A machine that consumes up to 350-500 kW is a rather rare occasion in hard-coating.
Besides the straight DC voltage, a hard-coating process can use DC voltage with a superimposed AC voltage (DC+AC voltage). This process was invented by Campbell and is described in the British Patent 716,554. The electrical power supply should provide for the superimposition of alternating and direct currents, usually up to 100 volts each, in order to form hard surface layers up to 250 microns. The current density is recommended to maintain at above 5 A/dm.sup.2 and may be reduced toward the end of the process to improve adhesion.
The ability of using lower DC voltage component levels during hard-coating by DC+AC voltage was discovered by Lerner et al. and described in the U.S. Pat. No. 4,128,461. In the initial period of 1 to 8 minutes the DC voltage component is raised to about 10 volts and then is raised at a rate of about 1/2 volt per minute to a level within the range of 14 to 19 volts. Upon reaching that level, the DC and AC voltage components are held constant for a dwell period of at least 5 minutes, and then raised again for the remainder of the first hour up to 30, 40 or 50 volts.
Hard-coating by DC+AC can be conducted even at lower DC voltage levels. The final DC voltage component may be in the range from 14 to 80 volts as it was described in the U.S. Pat. No. 4,133,725 by Lerner et al.
A circuit for producing a DC voltage with superimposed AC voltage of industrial frequency is described in the U.S. Pat. No. 4,170,739 by Fruzstajer and Lerner. As illustrated in FIG. 1, the patent teaches to use in a DC+AC power supply a single phase or a multiphase transformer primary (23) of which is coupled with suitable voltage control device (22) such as saturable core reactor, semiconductor control rectifier, or autotransformer. Secondary (24) of the transformer has two types of windings: ordinary and unbalancing windings. All these windings are star-connected. An ordinary winding is used exclusively for supplying AC voltage to system (25) of rectifying circuit elements, whereas an unbalancing winding is used mainly for supplying an to unbalancing AC voltage to terminals (27 and 28) of load First load terminal (27) is connected to a system of rectifying circuit elements (25) and second load terminal (28) is connected to the unbalancing winding of the transformer so that a DC voltage component plus an AC voltage component are provided across the load. The maximum of the AC voltage is achieved when the second load terminal is moved away from the central point of the transformer secondary and connected to another end of the unbalancing winding. The wave-form of the AC component becomes close to sinusoidal with the help of coupling capacitor (29) connected between first load terminal (27) and the central point of the transformer secondary.
More specifically, the patented DC+AC power supply in the preferred embodiment is an unbalanced three-phase non-linear circuit with a floating center point in the primary of a three-phase transformer. The Y-connected windings of the transformer secondary coupled with diodes and with specifically connected capacitor generates DC voltage with a super-imposed close-to-sinusoidal AC voltage. This DC+AC voltage is supplied to aluminum articles which are a combination of resistive and capacitive loads. Said combination changes its parameters during the anodizing run. The circuit is self-controlling: as the oxide film builds up and its capacitive resistance and the active resistance to the current increases, the voltages across the transformer windings in the primary and in the secondary are automatically changed so that the DC voltage component across the tank gradually increases and the current through the tank gradually decreases. The degree of the voltage increase and of the current decrease depends on the composition of the aluminum alloy and on the concentration and the temperature of the electrolyte.
The larger the DC current output of the DC+AC power supply, the larger the power that dissipates in the tank while forming the oxide film on aluminum articles. 3000 A of direct current component at 15-20 VDC would generate about 60 kW of heat in the tank.
It was discovered from practice that the patented schematic has a threshold of about 100 kW beyond which it becomes very difficult to provide the needed DC output and the reliable operation of the power transformer without overheating it. Therefore, the circuitry taught by said patent is not feasible for manufacturing DC+AC power supplies for tens of kilo-amperes needed for architectural anodizing which demands dissipation of up to 500 kW and more in the anodizing tank.
Returning now to the straight DC architectural anodizing, I will estimate energy consumption during this process in a water solution of sulfuric acid. I will consider a rather habitual example of a 20,000-ampere-architectural-anodizing machine which is fed by a DC power supply that controls the voltage in the 0 to 25 VDC range. The process is conducted at a room temperature. We will assume that the average voltage during a run equals 20 VDC. Energy consumption for a 22-hour-day is equal to EQU 20 V*20,000 A*22 hr=8,800 kWhr
A chiller, needed to maintain room temperature of the tank electrolyte, consumes about a quarter of the energy spent on anodizing, or 2800 kWhr. Total energy consumption during a day then equals 11,000 kWhr. Annual energy consumption (250 days) equals 2,750,000 kWhr at a cost of over $400,000 if we assume the $0.15/kWhr rate.
Architectural anodizing in organic acids, such as sulphosalicylic acid, needs twice or thrice higher voltage levels than in conventional anodizing. It means that the energy cost increases proportionally.